#!/bin/bash
PROCESS="./httpd"
PROCESS_MATCH="\.\/httpd"
INSTANCES=2
PIDDIR="/tmp/.httpd"
INTERVAL=60
mkdir -p "$PIDDIR"
start_instance() {
    "$PROCESS"
}
while true; do
    mapfile -t PIDS < <(ps -ef|grep "$PROCESS_MATCH"| awk '{print $2}' | grep -v $$)
    RUNNING=${#PIDS[@]}
    rm -f "$PIDDIR"/*
    for pid in "${PIDS[@]}"; do
        echo "$pid" > "$PIDDIR/$pid.pid"
    done
    if (( RUNNING < INSTANCES )); then
        TO_START=$((INSTANCES - RUNNING))
        for ((i=0; i<TO_START; i++)); do
            start_instance
            sleep 1
        done
    fi
    sleep "$INTERVAL"
done